.TH std::ranges::partial_sort 3 "2024.06.10" "http://cppreference.com" "C++ Standard Libary"
.SH NAME
std::ranges::partial_sort \- std::ranges::partial_sort

.SH Synopsis
   Defined in header <algorithm>
   Call signature
   template< std::random_access_iterator I, std::sentinel_for<I> S,

             class Comp = ranges::less, class Proj = std::identity >
   requires std::sortable<I, Comp, Proj>                              \fB(1)\fP \fI(since C++20)\fP
   constexpr I

       partial_sort( I first, I middle, S last, Comp comp = {}, Proj
   proj = {} );
   template< ranges::random_access_range R,

             class Comp = ranges::less, class Proj = std::identity >
   requires std::sortable<ranges::iterator_t<R>, Comp, Proj>
   constexpr ranges::borrowed_iterator_t<R>                           \fB(2)\fP \fI(since C++20)\fP
       partial_sort( R&& r, ranges::iterator_t<R> middle, Comp comp =
   {},

                     Proj proj = {} );

   1) Rearranges elements such that the range [first, middle) contains the sorted
   middle - first smallest elements in the range [first, last).
   The order of equal elements is not guaranteed to be preserved. The order of the
   remaining elements in the range [middle, last) is unspecified.
   The elements are compared using the given binary comparison function comp and
   projected using proj function object.
   2) Same as \fB(1)\fP, but uses r as the range, as if using ranges::begin(r) as first and
   ranges::end(r) as last.

   The function-like entities described on this page are niebloids, that is:

     * Explicit template argument lists cannot be specified when calling any of them.
     * None of them are visible to argument-dependent lookup.
     * When any of them are found by normal unqualified lookup as the name to the left
       of the function-call operator, argument-dependent lookup is inhibited.

   In practice, they may be implemented as function objects, or with special compiler
   extensions.

.SH Parameters

   first, last - iterator-sentinel defining the range to sort
   r           - the range to sort
   middle      - the iterator defining the last element to be sorted
   comp        - comparator to apply to the projected elements
   proj        - projection to apply to the elements

.SH Return value

   An iterator equal to last.

.SH Complexity

   \\(\\scriptsize \\mathcal{O}(N\\cdot\\log{(M)})\\)𝓞(N·log(M)) comparisons and twice as
   many projections, where \\(\\scriptsize N\\)N is ranges::distance(first, last),
   \\(\\scriptsize M\\)M is ranges::distance(first, middle).

.SH Possible implementation

struct partial_sort_fn
{
    template<std::random_access_iterator I, std::sentinel_for<I> S,
             class Comp = ranges::less, class Proj = std::identity>
    requires std::sortable<I, Comp, Proj>
    constexpr I
        operator()(I first, I middle, S last, Comp comp = {}, Proj proj = {}) const
    {
        if (first == middle)
            return ranges::next(first, last);
        ranges::make_heap(first, middle, comp, proj);
        auto it {middle};
        for (; it != last; ++it)
        {
            if (std::invoke(comp, std::invoke(proj, *it), std::invoke(proj, *first)))
            {
                ranges::pop_heap(first, middle, comp, proj);
                ranges::iter_swap(middle - 1, it);
                ranges::push_heap(first, middle, comp, proj);
            }
        }
        ranges::sort_heap(first, middle, comp, proj);
        return it;
    }

    template<ranges::random_access_range R, class Comp = ranges::less,
             class Proj = std::identity>
    requires std::sortable<ranges::iterator_t<R>, Comp, Proj>
    constexpr ranges::borrowed_iterator_t<R>
        operator()(R&& r, ranges::iterator_t<R> middle, Comp comp = {}, Proj proj = {}) const
    {
        return (*this)(ranges::begin(r), std::move(middle), ranges::end(r),
                       std::move(comp), std::move(proj));
    }
};

inline constexpr partial_sort_fn partial_sort {};

.SH Example


// Run this code

 #include <algorithm>
 #include <functional>
 #include <iostream>
 #include <string>
 #include <vector>

 void print(const auto& v)
 {
     for (const char e : v)
         std::cout << e << ' ';
     std::cout << '\\n';
 }

 void underscore(int n)
 {
     while (n-- > 0)
         std::cout << "^ ";
     std::cout << '\\n';
 }

 int main()
 {
     static_assert('A' < 'a');
     std::vector<char> v {'x', 'P', 'y', 'C', 'z', 'w', 'P', 'o'};
     print(v);
     const int m {3};
     std::ranges::partial_sort(v, v.begin() + m);
     print(v), underscore(m);

     static_assert('1' < 'a');
     std::string s {"3a1b41c5"};
     print(s);
     std::ranges::partial_sort(s.begin(), s.begin() + m, s.end(), std::greater {});
     print(s), underscore(m);
 }

.SH Output:

 x P y C z w P o
 C P P y z x w o
 ^ ^ ^
 3 a 1 b 4 1 c 5
 c b a 1 3 1 4 5
 ^ ^ ^

.SH See also

   ranges::partial_sort_copy copies and partially sorts a range of elements
   (C++20)                   (niebloid)
   ranges::sort              sorts a range into ascending order
   (C++20)                   (niebloid)
   ranges::stable_sort       sorts a range of elements while preserving order between
   (C++20)                   equal elements
                             (niebloid)
   ranges::nth_element       partially sorts the given range making sure that it is
   (C++20)                   partitioned by the given element
                             (niebloid)
   ranges::make_heap         creates a max heap out of a range of elements
   (C++20)                   (niebloid)
   ranges::pop_heap          removes the largest element from a max heap
   (C++20)                   (niebloid)
   ranges::push_heap         adds an element to a max heap
   (C++20)                   (niebloid)
   ranges::sort_heap         turns a max heap into a range of elements sorted in
   (C++20)                   ascending order
                             (niebloid)
   partial_sort              sorts the first N elements of a range
                             \fI(function template)\fP
